Distorted Ultrasmall TiO ₂ Nanoparticles as Efficient Visible Light Photocatalyst

ABSTRACT Developing efficient and low‐cost visible light photocatalysts remains a fundamental challenge. Titanium dioxide (TiO ₂ ), although stable and abundant, suffers from a wide bandgap and limited surface area. In this study, visible light active ultrasmall approximately 2 nm (∼2 nm) TiO ₂ nanoparticles were synthesized to overcome these limitations. Unlike conventional post‐annealing routes that promote particle growth, this visible light‐active ultrasmall TiO ₂ was directly obtained through a continuous flow‐hydrothermal process with precise temperature control and sub‐second reaction time. The ultrasmall (∼2 nm) TiO ₂ structure reveals the highly distorted lattice structure. The high lattice distortion, which contains an undercoordinated Ti atom, induces the localized states that modify the band structure of TiO ₂ and decrease its activation energy to the visible light region. The resulting ∼2 nm TiO ₂ exhibits nearly threefold higher photocatalytic activity for NO _x removal under visible irradiation, opening a new direction for designing ultrasmall oxide nanomaterials for next‐generation photocatalysis.